XIII.
MICROWAVE AND QUANTUM MAGNETICS
Academic and Research Staff
Prof. F.R. Morgenthaler
Prof. R.L. Kyhl
Dr. T. Bhattacharjee
D.A. Zeskind
Graduate Students
D. Fishman
L. Hegi
L.M. Itano
D.D. Stancil
N.P. Vlannes
F.R. Morgenthaler
Our objective is to develop an understanding of magnetostatic and magnetoelastic wave phenomena and to employ them to create novel device concepts useful
for microwave signal-processing applications.
Several years ago, work at M.I.T. by Zeskind and Morgenthaler, and subsequently, Cooley proved the feasibility of efficiently coupling, with microstrip
lines, to very high-Q tunable microwave resonances in rectangular slabs and disks
of single-crystal yttrium iron garnet (YIG).
Our interpretation of these resonances (potentially useful for microwave filtering) is that localized magnetic-mode patterns are formed in which the resonant
energies are guided or confined by regions of dc magnetic-field gradient within the
crystal. In the first experiments such gradients arose naturally from the nonuniform-shape demagnetizing fields; in subsequent work, we have created prespecified
gradients with shaped pole pieces designed by field-synthesis techniques.
This work has two motivations. First, we want to learn whether or not it is
feasible to create a multiplicity of tunable, high-Q resonances within the same
crystal, whether all at the same frequency or separated by preset amounts. Such a
structure could serve as an integrated magnetostatic filter circuit. Second, renewed interest in magnetostatic waves for signal-processing applications in the
frequency ranges above L-band (where surface acoustic wave (SAW) device fabrication
is difficult) has prompted us to inquire whether frequency dispersion, group velocity, and mode energy distribution can be controlled in a practical manner.
PR No. 123
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
1. MAGNETOSTATIC MODES BOUND BY DC H-FIELD GRADIENTS
Joint Services Electronics Program (Contracts DAAG29-78-C-0020 and
DAAG29-80-C-0104)
United States Air Force (Contract F19628-79-C-0047)
Frederic R. Morgenthaler, Robert L. Kyhl, Tushar Bhattacharjee,
Dale A. Zeskind, Daniel D. Stancil
Our interest in controlling magnetostatic waves and modes by means of dc field
gradients dates from the experimental observation of localized high-Q resonance
in single-crystal yttrium iron garnet (YIG) reported by Zeskind and Morgenthaler. 1
These and subsequent experiments have recently been reviewed in an invited paper2
presented at the Third International Conference on Ferrites held September 29October 2, 1980 in Kyoto, Japan. We also have extended our previous theoretical
treatment 3 of two-dimensional magnetostatic modes of single-domain thin ferrite
circular disks or annular rings, when the dc magnetic field is normal to the plane
and varies radially.
In the absence of a radial gradient, all of the modes of a solid disk have
circularly polarized rf h-fields with zero volume divergence. These modes are
strongly influenced by the magnetic pole distribution on the edge of the disk and
the rf energy becomes progressively concentrated near the rim as the mode index
increases.
The volume divergence of certain modes can change so dramatically that selective localization or expulsion of the energy occurs. The sense of polarization can
also reverse. In certain cases, the volume divergence of the rf magnetization can
become infinite (in the lossless-exchangeless approximation) at a certain interior
radius, rx.
The magnetic pole distribution at this "virtual-surface" thus resem-
bles that of a true surface and can serve to guide and localize the mode.
Such modes have now been reanalyzed in terms of the polarization factor of the
rf magnetization. Analytic solutions of that factor are continuous through the
"virtual-surface" and are given for the two-dimensional modes associated with
special field profiles.
Consideration of the radial impedance vs dc field plane
provides a great deal of insight into the character of both boundary-dominated and
gradient-dominated modes.
PR No. 123
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
The rf field patterns for a typical mode are shown in Fig. XIII-1 in which
the "virtual-surface" is shown as a dotted circle.
All three fields rotate syn-
chronously at the mode frequency.
(b) h-field locus
(a) n-field locus
(c) b-field Inc,,
Fig. XIII-1.
Loci of m-, h-, and b-field lines for the lowest order
"virtual-surface" mode when Hz(r)/M = 0.3 + 1.095 (r/R)4
All patterns rotate at the mode frequency w. The "virtualsurface" (shown dotted) occurs at r/R = 0.385.
Notice that the rf magnetization reverses direction in passing through the
critical radius constituting, in effect, a "microwave domain" with a 180-degree
rf "domain wall."
The role of exchange at a "virtual-surface" has been discussed by Morgenthaler 4
PR No. 123
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
and recently by Stancil 5 in a paper presented at the 1980 Intermag Conference held
in Boston during April. The effects of magnetic loss on the spin-wave component
of the mode potential and the effect on the overall mode pattern have also been
treated.6
In the absence of both exchange and loss, the magnetostatic potential has a
logarithmic singularity at the critical radius. Consequently, the radial h-field
and tangential E-field will there become infinite. Either exchange or loss will
remove the singularity. The cusps in the b-field are similarly removed making
finite all the rf fields and energy densities.
References
1. D.A. Zeskind and F.R. Morgenthaler, "Localized High-Q Ferromagnetic Resonance
in Nonuniform Magnetic Fields," IEEE Trans. on MAG, Vol. MAG-13, No. 5, September 1977.
2. F.R. Morgenthaler, "Novel Devices Based upon Field Gradient Control of Magnetostatic Modes and Waves," 3rd International Conference on Ferrites, Tokyo,
Japan, September 29-October 2, 1980.
3. F.R. Morgenthaler, "Bound Magnetostatic Waves Controlled by Field Gradients in
YIG Single Crystals and Epitaxial Films," IEEE Trans. on MAG, Vol. MAG-14,
September 1978, pp. 806-810.
4. F.R. Morgenthaler, "Two-Dimensional Magnetostatic Resonances in a Thin Film
Disk Containing a Magnetic Bubble," presented at the 1978 Conference on Magnetism and Magnetic Materials, Cleveland, Ohio, November 14-17, 1978.
5. D.D. Stancil, "Magnetostatic Waves in Nonuniform Bias Fields Including Exchange
Effects," IEEE Trans. on MAG Conference Proceedings, September 1980.
6. F.R. Morgenthaler, "Synthesis of Magnetostatic Waves and Modes Using Nonuniform Bias Fields," 1980 Ultrasonics Symposium, Boston, November 1980, to be
published in Conference Proceedings.
2. OPTICAL DETECTION OF MAGNETOSTATIC RESONANCES
Joint Services Electronics Program (Contracts DAAG29-78-C-0020 and
DAAG29-80-C-0104)
Nickolas P. Vlannes, Frederic R. Morgenthaler
The doctoral thesis of N.P. Vlannes will attempt optical detection of localized magnetostatic resonances in thin LPE films of YIG. At infrared frequencies,
the magneto-optical response of YIG can be modeled by a dielectric tensor that is
PR No. 123
(XIII.
dependent upon magnetization.
MICROWAVE AND QUANTUM MAGNETICS)
Thus one can probe for the characteristics of the
magnetization.
The experimental setup comprising a Spectra Physics 125 Laser tuned to
1150 nm with lens elements for focusing the beam and optical detectors capable of
responding to 2-GHz modulation rates is essentially complete.
A list of our goals follows.
1. Design and construction of an optical probing system to examine the microwave magnetization of magnetostatic modes of YIG;
2. Design and construction of an induction probing system to sense fringing
microwave magnetic fields;
3. Use of the probing systems to probe for the respective rf fields of YIG,
particularly under nonuniform bias-field conditions;
4. Conduct experiments on the effects of different antenna geometries and
antenna orientations relative to the YIG substrate in regard to signal
processing character and field patterns;
5. Test the microwave magnetic effects of various nonuniform bias fields
induced by varied-magnet pole-piece geometries.
3. MODE SYNTHESIS
United States Air Force (Contract F19628-79-C-0047)
Daniel Fishman, Frederic R. Morgenthaler
Nonuniform magnetic bias field can control magnetostatic waves and modes in
ferrite thin films. Gradients in either the field magnitude, direction or both
can be employed to synthesize wave or mode spectra to prespecified characteristics.
Parameters affected include frequency, rf energy distribution, impedance,
velocity of energy circulation, and the threshold governing the onset of nonlinear
effects due to parametrically unstable spin waves.
The important case of a thin-film disk that is surrounded by free space and
magnetized to saturation along the normal to its plane was analyzea in the 1980
Intermag Conference Proceedings 1 for the conditions of weak-to-moderate radial
gradients or arbitrary form:
PR No. 123
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
Hz(r)
A + Z B r2n
n=l
The non-"virtual-surface" modes analyzed are quasi-two-dimensional in that the
rf magnetization is assumed to be without appreciable thickness variation; the
fringing magnetic fields are, of course, three-dimensional because the disk is surrounded by free space.
The results allow one to synthesize modes with prespecified characteristics,
such as frequency, energy distribution, velocity of energy-circulation impedance
level, and power threshold for nonlinear behavior.
If it is desired to create a spectrum with the resonance frequencies separated
by prespecified amounts or if one wishes to control the velocity of energy circulation V of individual modes, the independent constants B can be adjusted and the
n
required field H (r) synthesized.
E
Control of V not only affects the group delay of signals propagating through
the mode but also the total energy, E, of the mode in terms of the signal power
P .
The normally slow energy circulation that occurs when B = 0 can either be
speeded up or slowed down. In the latter event, the direction of net power flow
can even be reversed. Near the balance point where V = 0, E becomes very large,
for a fixed value of P . Because nonlinear behavior, due to parametric spin-wave
instabilities, occurs when the energy density of the mode reaches a critical value,
it follows that the threshold power of the limiting level should be gradientcontrollable.
It is useful to realize that VE can be forced to be independent of m over some
range of m. This also suggests that magnetostatic plane-wave propagation can be
made precisely nondispersive over a predetermined bandwidth.
In Fig. XIII-2, we give the theoretical curves that result from forcing VE
to be constant for the first three modes (m = 1,2,3)
by means of Bl,B
PR No. 123
2 ,B3.
t*
Z--A
-a
X (U$iTS= i- i
--
- '
-NIT
Z -i
UNITS=
-X 1i
:
+
E-2)
Y <UNITS=
8.18
1.88
1.C5
0 .00
!-------
------------I-I
i
4
6.85+
,
+
8.15*
6.'D J.
0.4 0 *
0.i *5
8.50*
8.55t,
18
ii
12
8.68 *.
8.65+
4
i- *+
i4 *
i5 +
i6 *
a.78+
B.75+
8.8 *
17 *
17 *
e.9 ,
8.95;
1.88;
,,
Fig. XIII-2.
*
284
The normalized mode frequency (a) and prespecified energy frequency (b) both
plotted vs mode number, together with the required field profile (c) plotted
vs radius.
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
References
1. F.R. Morgenthaler, "Synthesized Magnetostatic Resonances in a Nonuniformly
Biased Thin Disk without Conducting Boundaries," IEEE Trans. on MAG Conference
Proceedings, September 1980.
4. NEW TECHNIQUES TO GUIDE AND CONTROL MAGNETOSTATIC WAVES
United States Air Force (Contract F19628-79-C-0047)
Joint Services Electronics Program (Contracts DAAG29-78-C-0020 and
DAAG29-80-C-O104)
Daniel D. Stancil, Frederic R. Morgenthaler, Dale A. Zeskind
The prospect of guiding magnetostatic waves is of considerable interest because of possible device applications. Such guided waves might be used to increase
the delay time realizable on a given-size sample by meandering the path, or to make
a resonator by guiding the waves along a closed loop. In addition, controlling
the coupling between adjacent waveguides could make possible signal-routing devices
such as directional couplers.
Guiding magnetostatic surface waves (MSSWs) is complicated by the fact that
MSSW propagation is only possible in one direction on a given surface when the
applied in-plane bias field is uniform; turns of 900 would normally require con-
Permalloy
strips
r,,r,~uvcllr
htrnt ulr,
vvv
.- YIG film
-Alumina
Ground plane
- HDC
x
Fig. XIII-3.
PR No. 123
Basic experimental configu ration.
version to backward-volume
waves.
It should be possible to
overcome this difficulty by
employing gradients that arise
from a change in the direction
As an exof the bias field.
ample, consider a YIG film that
is covered with permalloy containing a slot of controlled
width. If the permalloy is at
a different magnetostatic potential on either side of the slot,
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
the dc magnetic field will be parallel to the film in the region underneath the
slot but normal to the film surface in those portions directly underneath the
permalloy.
The in-plane fields permit magnetostatic surface wave propagation; the normal
fields do not.
Therefore the surface-wave energy should be localized under the air-filled
slot rather than under the conducting permalloy; eddy-current dissipation is thereby minimized.
Notice also that if the entire field is normally directed, the surface wave
disappears completely.
Taken together, these factors should allow novel control
of the surface-wave channel. Notice,
too, that the width of the slot will
strongly affect the speed of propa-
0
g20-
gation and more than one slot can
Lo
be placed side-by-side to provide
coupled-wave possibilities. The slots
1
can be curved or even meandered with
the in-plane field automatically ad^
_justing
paths can be closed themselves to
form resonant tracks that are single
r __
-
0.35
01
direction to maintain the
surface-wave conditions. Such curved
O.3
or multiple.
As a first step toward evaluating this approach, we have con-
0.25
O.2I
2.0
2.4
2.8
3.2
Frequency (GHz)
Fig. XIII-4.
MSSW transmission spectra as a function of
the spacing between permalloy strips. The
fine structure near the high-frequency end
is due to interference with the EM feedthrough.
PR No. 123
ducted preliminary experiments and
have recently reported1 the observation of magnetostatic surface waves
(MSSWs) in a rectangular YIG film
placed between strips of permalloy
and in the plane of the strips. In
such a configuration, shown in Fig.
XIII-3, the dc magnetic field between
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
the permalloy strips is primarily in-plane with a minimum halfway between the
strips. In the presence of such a gradient, a series of discrete modes appears
below the low-frequency end of the continuous MSSW band as shown in Fig. XIII-4.
We report here the results of pulse-time-delay measurements indicating that these
modes travel several times faster than normal MSSW modes, in agreement with previous cw measurements.
Fig. XIII-5.
g
O
200
150
nS
E
Cr
Surface wave precursors. (a) Precursor due to leading edge of a pulse
centered at 2.000 GHz with H = 300 Oe
and bottom of the band measured to be
2.388 GHz. Nominal film thickness s =
5 microns. (b) Theoretical precursor
based on H = 300 Oe, 4 Ms = 2122 G,
s = 3.19 microns, and L = 1 cm.
(b)
130
140
150
160
170
nS
We also reported 2 the experimental observation of MSSW precursors in both
uniform and nonuniform fields. An approximate theory for the precursors in a uniform field has been developed which agrees well with the measurements. The comparison is shown in Fig. XIII-5.
PR No. 123
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
References
1. D.D. Stancil and F.R. Morgenthaler, "Magnetostatic Surface Modes in a Thin Film
with Nonuniform In-Plane Fields," IEEE Trans. on Magnetics, Vol. MAG-16, No. 5,
September 1980.
2. D.D. Stancil and F.R. Morgenthaler, "The Effects of Nonuniform In-Plane Fields
on the Propagation Characteristics of Magnetostatic Surface Waves," 1980 Ultrasonics Symposium, November 1980, to be published in Conference Proceedings.
5. MAGNETOSTATIC WAVE-DISPERSION THEORY
United States Air Force (Contract F19628-79-C-0047)
Joint Services Electronics Program (Contracts DAAG29-78-C-0020 and
DAAG29-80-C-0104)
Frederic R. Morgenthaler, Tushar Bhattacharjee, Daniel D. Stancil
In our prior analyses of nonuniform magnetostatic modes in thin films, the
bias field was assumed to be directed normal to the plane and the modes considered
were quasi-two-dimensional in that thickness variations of the rf magnetization
were ignored. This restriction conformed to the geometry of several experimental
configurations but our most recent studies1 have involved films with nonuniform
in-plane bias and, consequently, we have now developed a general theory, based
upon coupled-integral equations, that can deal with surfacelike modes in one or
more concentric thin-film rings when the bias and any material nonuniformities
(such as variation of the saturation magnetization) are purely radial. We have
also treated the analogous integral equations based upon Cartesian coordinates for
film configurations that involved one or more rectangular strips. Exchange effects
and thickness variations of the bias field and material parameters are not included
in the model but, because surface-wave character is expected, such variations of
the rf fields are permitted.
The boundary conditions required at the film surfaces are built directly into
these equations; the transverse conditions enter through the components of the
Polder susceptibility tensor.
The equations govern a very wide range of physical situations.
For example,
coupling between modes circulating on two concentric annular rings spaced by an
air gap can be handled as can the analogous problem involving two ferrite strips.
Computer algorithms for numerical solutions of this class of eigenfrequency
PR No. 123
(XIII.
MICROWAVE AND QUANTUM MAGNETICS)
problem are currently being developed.
References
1. F.R. Morgenthaler, "Magnetostatic Surface Modes in Nonuniform Thin Films with
In-Plane Bias Fields," 26th Conference on Magnetism and Magnetic Material,
Dallas, Texas, November 1980.
6. MAGNETOELASTIC WAVES AND DEVICES
United States Air Force (Contract F19628-79-C-0047)
Joint Services Electronics Program (Contracts DAAG29-78-C-0020 and
DAAG29-80-C-0104)
Frederic R. Morgenthaler, Leslie M. Itano
A microwave yttrium-iron-garnet (YIG) delay line can be made to provide linearly dispersive delay with instantaneous bandwidth in excess of 1 GHz in C-band.
The dispersion of the device is based on the properties of spin and elastic waves
in magnetically biased YIG, a ferrimagnetic material. In particular, linear dispersion can be achieved by first specifying a particular dc magnetic-field profile
along the axis of a YIG rod, and then designing the necessary pole pieces with
the aid of a computer.
The first generation of thin-film antennae were fabricated, but the results
were disappointing. A second set of designs is currently being processed, and
modifications of the test fixture are being made that are expected to increase
the focusing of the magnetostatic backward wave into the magnetoelastic beam. The
hoped-for result will be an increase in coupling efficiency and consequent reduction in the insertion loss of the magnetoelastic wave signal.
PR No. 123